1. Field of the Invention
The present invention relates to data communication. More particularly, the present invention relates to a novel and improved method and apparatus for forward link rate scheduling of high speed data transmission in a communication system having a variable data transmission rate.
2. Description of the Related Art
A modern day communication system is required to support a variety of applications. One such communication system is a code division multiple access (CDMA) system which conforms to the “TIA/EIA/IS-95A Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System”, hereinafter referred to as the IS-95A standard. The CDMA system allows for voice and data communications between users over a terrestrial link. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, entitled “SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS”, and U.S. Pat. No. 5,103,459, entitled “SYSTEM AND METHOD FOR GENERATING WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM”, both assigned to the assignee of the present invention and incorporated by reference herein.
The IS-95A standard is designed to optimize voice communication and many important system design parameters are selected to achieve that goal. For example, since time delay between speakers cannot be tolerated, processing delays are sought to be minimized. Each user is allocated a traffic channel capable of carrying speech data for the duration of the call. Upon termination of the call, the traffic channel becomes available for another user.
In accordance with the IS-95A standard, each traffic channel is designed to support a symbol rate of 19.2 Ksps. Using a rate 1/2 convolutional encoder, the data rate of each traffic channel approaches 9.6 Kbps. Although not specified by the IS-95A standard, higher data rates can by supported by the use of other code rates. For example, a data rate of 14.4 Kbps is achieved by using a rate 1/2 convolutional encoder and deleting two out of every eight symbols, to obtain a punctured rate 3/4 convolutional encoder.
The CDMA system must work within the pre-existing frequency allocation in the cellular band. By design, a CDMA system which conforms to the IS-95A standard is allotted a 1.2288 MHz bandwidth to fully utilize the cellular band. The forward link refers to transmission from a cell to the remote stations. On the forward link, the 1.2288 MHz bandwidth is divided into 64 code channels, with each code channel having a capacity of 19.2 Ksps. Most of the code channels are defined as traffic channels which are allocated, upon demand, to users for voice communication. Some code channels are defined as paging channels used for paging and messaging between the cell and the remote stations. Several code channels, such as the pilot and sync channels, are reserved for system overhead.
In the CDMA system, users communicate with one another through remote stations which, in turn, communicate with each other through one or more base stations. In this specification, base station refers to the hardware with which the remote stations communicate. Cell refers to the hardware or the geographic coverage area, depending on the context in which the term is used.
In the CDMA system, communications between users are conducted through one or more cells which are serviced by base stations. A first user on one remote station communicates to a second user on a second remote station, or a standard telephone, by transmitting voice data on the reverse link to a cell. The cell receives the voice data and can route the data to another cell or a public switched telephone network (PSTN). If the second user is on a remote station, the data is transmitted on the forward link of the same cell, or a second cell, to the second remote station. Otherwise, the data is routed through the PSTN to the second user on the standard phone system. In IS-95A systems, the forward link and the reverse link are allocated separate frequency and are independent of one another.
The remote station communicates with at least one cell during a communication. CDMA remote station are capable of communicating with multiple cells simultaneously during soft handoff. Soft handoff is the process of establishing a link with a new cell before breaking the link with the previous cell. Soft handoff minimizes the probability of dropped calls. The method and system for providing a communication with a remote station through more than one cell during the soft handoff process are disclosed in U.S. Pat. No. 5,267,261, entitled “MOBILE ASSISTED SOFT HANDOFF IN A CDMA CELLULAR TELEPHONE SYSTEM,” assigned to the assignee of the present invention and incorporated by reference herein. Soft handoff impacts various aspects of the CDMA system design because considerations should be given to the status and capacity of each of the multiple cells involved in the soft handoff when a new allocation of resource is made.
The CDMA system is a spread spectrum communication system. The benefits of spread spectrum communication are well known in the art and can be appreciated by reference to the above cited references. Each code channel in the CDMA system can transmit up to 19.2 Ksps. The 19.2 Ksps is then spread over the entire 1.2288 MHz system bandwidth. The IS-95A CDMA system increases capacity by transmitting fewer bits, thereby using less power, when the user is not speaking. Since the forward link capacity between the cell and the remote station is limited by the maximum transmit power available for the cell, decreasing the transmit power during idle periods increases the forward link capacity.
The user on each remote station transmits at a different bit rate depending on the level of speech activity in the conversation of that user. A variable rate speech vocoder provides speech data at full rate when the user is actively speaking and at low rate during period of silence, e.g. pauses. The variable rate vocoder is described in detail in U.S. Pat. No. 5,414,796, entitled “VARIABLE RATE VOCODER,” assigned to the assignee of the present invention and incorporated by reference herein.
The forward link capacity for voice communication between the cell and the remote stations, as measured by the number of users supportable by the CDMA system, can be determined by the bit rate of the user on each remote station. This is because other parameters determinative of the forward link capacity are fixed by the system design or given. For example, the maximum transmit power available for each cell is limited by FCC regulations and also by the acceptable levels of adjacent cell interference. The transmit power required for a given symbol rate depends on the energy-per-bit-to-noise ratio (Eb/No) required by the remote station, the path loss (e.g. location of the remote station within the cell) and the noise level, all of which cannot be controlled. The Eb/No required to maintain the desired level of performance is dependent on the channel condition, e.g. fading. Finally, the CDMA system bandwidth of 1.2288 MHz is selected by design.
On the forward link, the required transmit power is also dependent on the orthogonality of the code channels. Walsh code spreading is used to achieve orthogonality of the forward link code channels. The orthogonality minimizes the interference between the code channels. This orthogonality is not preserved in a multipath environment and, as the result, the level of interference increase. The required transmit power is then increased to maintain the same operating Eb/No.
The amount of speech activity at any given moment is non-deterministic. Also, there is typically no correlation in the level of speech activities among users. Therefore, the total power transmitted from a cell to all users in that cell varies over time and can be approximated as a Gaussian distribution. During the period of time when the level of speech activities is high and the required transmit power exceeds the maximum transmit power available to the cell, each voice data bit is transmitted with less power than would be optimal. Since the path loss is fixed, the Eb/No drops. The lower Eb/No increases the probability of frame errors in the voice data received by the users. This event is known as an outage.
The number of users able to have access to the communication system is limited so that a predetermined frame error rate (FER) is maintained. Limiting the forward link capacity to maintain the predetermined FER has the effect of forcing the cell to transmit at less than full capacity, on the average, thereby under-utilizing the forward link capacity of the cell. In the worse case, up to half of the forward link capacity is wasted to maintain a headroom of up to 3 dB. The headroom is the difference between the maximum transmit power available to the cell and the average transmit power of the cell. The headroom is only utilized during the period when the speech activities of the users are high.
Data communication within the CDMA system has different characteristics than voice communication. For example, data communication is typically characterized by long period of inactivity, or low activity, punctuated by high bursts of data traffic. An important system requirement for data communication is the transmission delay required to transfer the burst of data. Transmission delay does not have the same impact in data communication as it does for voice communication, but it is an important metric for measuring the quality of the data communication system.
A method for transmitting data traffic in code channel frames of fixed size, wherein the data source provides data at a variable rate, is described in detail in U.S. Pat. No. 5,504,773, entitled “METHOD AND APPARATUS FOR THE FORMATTING OF DATA FOR TRANSMISSION”, assigned to the assignee of the present invention and incorporated by reference herein. Data is partitioned into data frames and each data frame may be further partitioned into data portions. The data portions are then encoded into code channel frames which are 20 msec wide. At the 19.2 Ksps symbol rate, each code channel frame contains 384 symbols. A rate 1/2, or a rate 1/2 punctured to obtain a rate 3/4, convolutional encoder is used to encode the data, depending on the application. Using a rate 1/2 encoder, the information rate is approximately 9.6 Kbps. At the 9.6 Kbps data rate, there are 172 data bits, 12 cyclic redundancy check (CRC) bits and 8 code tail bits per code channel frame.
High speed data transmission on the forward link can be achieved by concurrently transmitting data traffic over multiple code channels. The use of multiple code channels for data transmission is disclosed in U.S. patent application Ser. No. 08/656,649, entitled “METHOD AND APPARATUS FOR PROVIDING RATE SCHEDULED DATA IN A SPREAD SPECTRUM COMMUNICATION SYSTEM”, filed May 31, 1996, assigned to the assignee of the present invention and incorporated by reference herein.
The demand for the forward link continuously changes over time, in part due to variations in the level of voice activities. The inefficient use of the forward link can be improved by transmitting data traffic during the period of low voice activity. To avoid degradation in the quality of the voice communication, the data transmission should be dynamically adjusted to match the available forward link capacity of the cell.
In dealing with large sporadic bursts of data traffic, a system should be designed with the capability to transmit at high data rates and the ability to allocate the forward link resource to the users whenever requested, based on the availability of the resource. In a CDMA system, the design should address other existing system considerations. First, since voice communication cannot tolerate extensive delay, priority should be given to transmission of voice data over transmission of any data traffic. Second, since the amount of voice activity at any given moment is unpredictable, the forward link should be continuously monitored and the data transmission should be dynamically adjusted so that the forward link capacity is not exceeded. Third, since the user may be in soft handoff between multiple cells, the data transmission rate should be assigned based on the forward link capacity of each of the cells participating in the soft handoff. These and other considerations are addressed by the present invention.